Not necessarily .. in, for instance Sweden, the lowest number of babies are born nine months after Valentine's day.

Lots of sex during holidays in Sweden used to translate into lots of babies nine months later. But not anymore, according to a new study in demography from Stockholm University.

Between 1940 and 1999 in Sweden, births used to peak nine months after Midsummer and Christmas respectively. As abortion became more accessible, and contraception came more fully into play - especially for the unmarried - the seasonal patterns began to reflect more planning of when parents prefer to have their children. This result is clearly shown in a new study by demographers Johan Dahlberg and Gunnar Andersson. They used register data on live births between 1940 and 2012 in Sweden to study this phenomenon.

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“Less and less children are born at the end of the year. This means that November - nine months after Valentine’s day - is actually during the period of time when the least number of babies are born. The plausible reason is that parents are planning against having their babies at the end of the year – a pattern that can be seen in other countries as well,” says Gunnar Andersson, professor of demography at Stockholm University and co-author of the study.
In fact, the only seasonal variation left for births in Sweden are these low birth rates at the end of the year. This pattern has also gained in magnitude.

“Since Valentine’s Day is also a new phenomenon in Sweden – and limited to one day – it has absolutely no positive effect on birth rates in this country,” Andersson continues.
Previous studies have shown that children who are born late in the year have disadvantages in school compared with classmates born earlier during the same year, as they become junior in their year group.
“We believe the strong cut-off in births at the end of the year is because parents want to avoid giving their children a disadvantage in school,” says Dahlberg, researcher in demography and first author of the study.

"Valentine’s Day has absolutely no positive effect on birth rates in this country,” says Gunnar Andersson, professor in demography and head of the Stockholm University Demography Unit (SUDA). Image: Stockholm University